The present invention relates to means for effecting temperature-compensation of focal length in an optical assembly.
Certain optical assemblies such as those found in laser output scanners require a stable monochromatic collimated light beam provided by a laser diode and a collimating lens. For adequate optical performance, the beam source must maintain a predetermined beam quality over a wide ambient temperature range. In conventional apparatus, the beam source and lens are mounted in a mechanical structure that attempts to maintain the beam focal length while the apparatus undergoes temperature-induced structural changes. Hence, the athermalization (i.e., thermal compensation) is effected either passively or actively.
Passive compensation systems typically rely on the differences in coefficients of thermal expansions of the various elements in the optical system such that there is minimal net focus shift with temperature. For example, the conventional approach is to employ concentric tube systems, which, if constructed from common materials, are too large or bulky. U.S. Pat. No. 4,730,335 discloses a series of interlocking tubes each carrying a single optical element of an optically-pumped solid-state laser. Active compensation systems typically rely on active temperature control of the various elements in the optical system such that there is minimal temperature change. For example, a thermoelectric cooler is employed in the apparatus disclosed in U.S. Pat. No. 4,604,753 to stabilize the output power and wavelength of a laser diode beam source; U.S. Pat. Nos. 4,656,635 and 4,993,801 disclose a beam source wherein a thermoelectric cooler is employed to control the operating temperature of the entire head.
However, the accuracy of a passive system will depend upon the CTE of the materials chosen for the construction of certain components of the system. Thus, some passive designs cannot be manufactured because the desired CTE is simply not exhibited by the materials suited for fabricating the requisite system components, or because the CTE is available but tends to be unstable. Further, the accuracy of known active systems is often dependant upon the construction of complex components in exacting tolerances. The foregoing approaches have accordingly been found to be more costly and complex, and offer less precision and less range of adjustment, than is desired for certain applications.